The present invention relates to an apparatus for plotting information and more particularly relates to an apparatus for plotting information onto a continuous sheet of a recording medium.
Seismic surveys have long been used to search for subterranean minerals, e.g., oil and gas. To conduct such a survey, an impulse from an explosion or the like is generated at an originating point. As the impulse travels downward through the area under investigation, a series of signals is generated as the impulse is reflected off the various subterranean formations. These signals are detected and recorded at a plurality of points by geophones or the like which are spaced at predetermined distances from the originating point. This recorded data is then refined and processed along with data from other originating points and the resulting information is plotted to produce a visual representation called a seismic section.
A seismic section is normally plotted on photographic film or the like and in finished form is comprised of a plurality of common depth point traces positioned in a side-by-side relationship which, when viewed by an analyst, can disclose various characteristics of the area surveyed.
Various types of plotters have been proposed for plotting seismic sections. Probably the most common plotter presently in commercial use is the "drum type" plotter. This plotter uses a standard size sheet of film, e.g., 24 inches (60.96 cm.) by 36 inches (91.44 cm.), which is secured around a rotating cylinder or drum. A camera, e.g., cathode ray tube, is positioned above the drum so that processed data displayed on the face of the tube is optically focused onto the film lying under the camera. The camera is held stationary while the drum is rotated 360.degree.. This permits data to be exposed onto that portion of the film which passes under the camera. The drum then stops and the camera is automatically stepped over so that a new portion of the sheet of film is exposed under the camera. The drum is again rotated and the procedure is repeated until a seismic section is completed or until the camera has been stepped across the operational width of the drum.
When using a drum type plotter, it is not uncommon for some seismic sections to contain too many traces or have too great a spread for the section to be plotted on a single sheet of film. In these instances, the data is recorded on more than one sheet of film and these sheets are spliced together to form the completed section. This procedure, although satisfactory, is time consuming and requires multiple handlings to get a completed section.
Another type of plotter which has been proposed for preparing seismic sections is one where the seismic data is reproduced onto a continuous sheet of film or the like, e.g., see U.S. Pat. Nos. 2,976,107 and 3,588,911. In these continuous type plotters, the camera is mounted on a carriage which sweeps across an area of film which in turn is held stationary under the camera. When a sweep is completed, the film is advanced to expose a new area before another sweep is carried out. This procedure is continued until the seismic section is completed. This permits a section comprised of a large number of traces or having a large spread to be plotted in a single operation without changing or handling multiple sheets of the film.
In plotting seismic sections on a continuous type plotter, several aspects of the plotter must be considered to insure that the processed data is accurately transcribed onto the film so that there will be substantially no error in the finished visual representation of the data. One such aspect involves the precision control of the carriage as it sweeps across the film. Not only must the speed of the carriage be maintained at an exact, constant speed during the actual plotting of data, but, also, the acceleration and deceleration of the carriage as it starts and stops must be taken into account.
Another important aspect involves the proper feeding and positioning of the film as a section is being plotted. The film must be advanced an exact, defined increment before each sweep of the carriage so that the transcribed data does not overlap or have unwanted gaps therein. Also, the film must be fed in such a way that it always lies flat with respect to the camera to avoid distortions which could seriously affect the accuracy of the data on the finished seismic section.